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Well clients/users would login by going to the site. (i.e. domain.tld)

Staff would login going to the site/scp. (i.e. domain.tld/scp)

The default admin account used to be admin... but circa 1.7.x it was changed to whatever the installer set it up as during installation. If you have SQL access you could query the database for staf_id 1, which is always the admin account unless someone changed it.